Large substrates with electronically active components distributed over the extent of the substrate are used in a variety of electronic systems, for example imaging devices such as flat-panel liquid crystal or OLED display devices and in digital radiographic plates. Large substrates with electrically active components are also found in flat-panel solar cells.
The electronically active components on flat-panel substrates are typically formed by sputtering a layer of inorganic semiconductor material or by spin-coating a layer of organic semiconductor material over the entire substrate and processing the layer to form electronic components. However, such coatings typically may have relatively poor electronic characteristics. Inorganic semiconductor materials can be processed to improve their electronic characteristics; for example, amorphous silicon can be heat-treated to form polycrystalline silicon. In other processes, microcrystalline semiconductor layers can be formed by using an underlying seeding layer. These methods typically improve the electron mobility of the semiconductor, but the performance of the resulting layer may still be worse than is often desired or needed.
The substrate and layer of semiconductor material can be photo-lithographically processed to define electronically active components, such as transistors. Such transistors are known as thin-film transistors (TFTs) since they are formed in a thin layer of semiconductor material, typically silicon. The photo-lithographic processing typically requires high-resolution mask steps to pattern the semiconductor materials and metal interconnections on the substrate. In these devices, the substrate is often made of glass, for example, Corning Eagle® or Jade® glass designed for display applications.
However, these conventional thin-film techniques may have limitations. Despite processing methods used to improve the performance of thin-film transistors, such transistors may have a performance lower than the performance of conventional integrated circuits formed in mono-crystalline semiconductor material. Also, semiconductor material and active components may be used on only portions of the substrate, leading to wasted material and increased material and processing costs. The substrate materials may be limited by the processing steps that may be necessary to process the semiconductor material, as well as the photo-lithographic steps used to pattern the active components. For example, plastic substrates have a limited chemical and heat tolerance and typically do not readily survive photo-lithographic processing. Furthermore, the manufacturing equipment used to process large substrates with thin-film circuitry may require relatively high resolution and may be expensive.
In another manufacturing technique, a mono-crystalline semiconductor wafer may be employed as the substrate. While this approach can provide substrates with similar performance as integrated circuits, the size of such substrates may be limited, for example, to a 12-inch diameter circle, and the wafers are relatively expensive compared to other substrate materials such as glass or polymer.
In yet another approach, thin layers of semiconductor may be bonded to a substrate and then processed. Such a method is known as semiconductor-on-glass or silicon-on-glass (SOG) and is described, for example, in U.S. Patent Application Publication No. 2008/0224254, published Sep. 18, 2008. If the semiconductor material is crystalline, high-performance thin-film circuits can be obtained. However, the bonding technique can be expensive and the high-resolution processing equipment needed to form thin-film active components on large substrates may be expensive.
It is also known to provide relatively large integrated circuits in surface mountable packages that are directly adhered to a substrate. However, these integrated circuits are relatively large and additional layers may not be easily formed over the integrated circuits. Furthermore, electrical inter-connections to the surface-mountable package may require high-resolution patterning.
Publication number 11-142878 of the Patent Abstracts of Japan entitled “Formation of Display Transistor Array Panel” describes etching a substrate to remove it from a thin-film transistor array on which the TFT array was formed. TFT circuits formed on a first substrate can be transferred to a second substrate by adhering the first substrate and the TFTs to the surface of the second substrate and then etching away the first substrate, leaving the TFTs bonded to the second substrate. This method typically requires etching a significant quantity of material and risks damaging the exposed TFT array. Also, as with the other techniques discussed above, the patterned processing may require expensive, high-resolution equipment.
Another method of locating material on a substrate is described in U.S. Pat. No. 7,127,810. In this method, a first substrate carries a thin-film object to be transferred to a second substrate. An adhesive is applied to the object to be transferred or to the second substrate in the desired location of the object. The substrates are aligned and brought into contact. A laser beam irradiates the object to abrade the transferring thin film so that the transferring thin film adheres to the second substrate. The first and second substrates are separated, peeling the film in the abraded areas from the first substrate and transferring it to the second substrate. In one embodiment, a plurality of objects may be selectively transferred by employing a plurality of laser beams to abrade selected areas. Objects to be transferred can include thin-film circuits. Further processing, for example to provide electrical interconnections between the transferred objects, may require high-resolution processing.
U.S. Pat. No. 6,969,624 describes a method of transferring a device from a first substrate onto a holding substrate by selectively irradiating an interface with an energy beam. The interface is located between a device for transfer and the first substrate and includes a material that generates ablation upon irradiation, thereby releasing the device from the substrate. For example, a light-emitting device (LED) can be made of a nitride semiconductor on a sapphire substrate. The energy beam is directed to the interface between the sapphire substrate and the LED nitride semiconductor releasing the LED and allowing the LED to adhere to a holding substrate coated with an adhesive. The adhesive is then cured. These methods, however, may require the patterned deposition of adhesive on the object(s) or on the second substrate. Moreover, the laser beam that irradiates the object may be shaped to match the shape of the object and the laser abrasion can damage the object to be transferred. Furthermore, the adhesive cure takes time, which may reduce the throughput of the manufacturing system. Further processing, for example to provide electrical interconnections between the transferred objects, may also require high-resolution processing.
In another method for transferring active components from one substrate to another, described in “AMOLED Displays using Transfer-Printed Integrated Circuits” published in the Proceedings of the 2009 Society for Information Display International Symposium Jun. 2-5, 2009, in San Antonio, Tex., US, vol. 40, Book 2, ISSN 0009-0966X, paper 63.2 p. 947, small integrated circuits (chiplets) with connection pads formed on the chiplet surface are formed in a wafer and released from the wafer by etching beneath the circuits. A PDMS stamp is pressed against the wafer and the circuits adhered to the stamp. The circuits are then pressed against a substrate coated with an adhesive, adhered to the substrate, and the adhesive is subsequently cured. Subsequent photo-lithographic processes are used to form electrical wires over the substrate and on to the connection pads. However, the position and orientation of the chiplets resulting from the printing process can vary somewhat. Thus, the connection pads may need to be relatively large so that the wires formed by the photo-lithographic processing steps contact the contact pads. The relatively large connection pads can reduce the space available for circuits and circuit connections, and thus can reduce the functionality of the chiplets.